Hello again,

I was provided with the code as follows;

function [t, u] = chemostat(initConds)

tspan = [0; 80]; %This command defines the time interval of interest.\

u0=initConds; %This command tells MATLAB the initial condition. Note that u0 is a (1 x 3) vector\

%whose first entry is n(0), and second entry is c(0). If you are solving a system of more than\

%two equations, simply place the additional initial conditions into the vector u0.\

[t,u] = ode15s(@f,tspan,u0);

% % Option to plot within the function

for j = 1:length(t) %This for-loop extracts the solutions for each variable, n and c, from the matrix u. \

n(j,1) = u(j,1); %Specifically, the first column of u is n, the second column of u is c.\

c(j,1) = u(j,2);

end

figure;

plot(t,n,t,c) %This tells MATLAB to plot the solutions as a function of time.\

% --------------------------------------------------------------------------\

function dudt = f(t,u) %This commands defines the function du/dt = f(t,u). Remember that u is a matrix that contains S, I, and R.\

a1 = 2; %Now define the parameters.\

a2 = 2;

% dn/dt = a1*c/(1+c)*n - n

% dc/dt = -c/(1+c)*n-c+a2

dudt = [a1*u(2)/(1+u(2))*u(1) - u(1); -u(2)/(1+u(2))*u(1)-u(2)+a2]; %

Which, of course runs wonderfully.

I need to plot 4 subplots of this function with 4 different initial conditions. This is my code here:

n1 = 0.1;

c1 = 0.1;

p1 = [n1,c1];

g1 = chemostat(p1);

n2 = 0.5;

c2 = 10;

p2 = [n2,c2];

g2 = chemostat(p2);

n3 = 2;

c3 = 5;

p3 = [n3,c3];

g3 = chemostat(p3);

n4 = 1.25;

c4 = 0.5;

p4 = [n4,c4];

g4 = chemostat(p4);

figure

subplot(2,2,1)

plot(g1)

title('Initial Conditions (n,c) = (0.1,0.1)')

subplot(2,2,2)

plot(g2)

title('Initial Conditions (n,c) = (0.5,10)')

subplot(2,2,3)

plot(g3)

title('Initial Conditions (n,c) = (2,5)')

subplot(2,2,4)

plot(g4)

title('Initial Conditions (n,c) = (1.25,0.5)')

Can someone tell me why I'm getting 4 correct figures for each plot but the subplots are wrong?

Thanks! E

Answer by John
on 13 Oct 2016

Hello,

I modified the function to suppress the plot and output plotting variables.

function [t, n, c] = chemostat(initConds)

tspan = [0; 80]; %This command defines the time interval of interest.\

u0=initConds; %This command tells MATLAB the initial condition. Note that u0 is a (1 x 3) vector\

%whose first entry is n(0), and second entry is c(0). If you are solving a system of more than\

%two equations, simply place the additional initial conditions into the vector u0.\

[t,u] = ode15s(@f,tspan,u0);

% % Option to plot within the function

for j = 1:length(t) %This for-loop extracts the solutions for each variable, n and c, from the matrix u. \

n(j,1) = u(j,1); %Specifically, the first column of u is n, the second column of u is c.\

c(j,1) = u(j,2);

end

% figure;

% plot(t,n,t,c) %This tells MATLAB to plot the solutions as a function of time.\

% --------------------------------------------------------------------------\

function dudt = f(t,u) %This commands defines the function du/dt = f(t,u). Remember that u is a matrix that contains S, I, and R.\

a1 = 2; %Now define the parameters.\

a2 = 2;

% dn/dt = a1*c/(1+c)*n - n

% dc/dt = -c/(1+c)*n-c+a2

dudt = [a1*u(2)/(1+u(2))*u(1) - u(1); -u(2)/(1+u(2))*u(1)-u(2)+a2]; %

Here's the code you should try in a script.

n1 = 0.1;

c1 = 0.1;

p1 = [n1,c1];

[t1a, n1a, c1a] = chemostat(p1);

n2 = 0.5;

c2 = 10;

p2 = [n2,c2];

[t2a, n2a, c2a] = chemostat(p2);

n3 = 2;

c3 = 5;

p3 = [n3,c3];

[t3a, n3a, c3a] = chemostat(p3);

n4 = 1.25;

c4 = 0.5;

p4 = [n4,c4];

[t4a, n4a, c4a] = chemostat(p4);

figure

subplot(2,2,1)

plot(t1a,n1a,t1a,c1a)

title('Initial Conditions (n,c) = (0.1,0.1)')

subplot(2,2,2)

plot(t2a,n2a,t2a,c2a)

title('Initial Conditions (n,c) = (0.5,10)')

subplot(2,2,3)

plot(t3a,n3a,t3a,c3a)

title('Initial Conditions (n,c) = (2,5)')

subplot(2,2,4)

plot(t4a,n4a,t4a,c4a)

title('Initial Conditions (n,c) = (1.25,0.5)')

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